Ich problem!

my goldfish have had ich for weeks now I have been trying
everything!It was only a spot or two to begin with which I would use
"ick Clear" Which was taking care of it for a couple of days But then
the spots would return It's a 30 gal. tank 4 fish all about 2inches. I
have live plants all the time,2 mystery snails I salt regularly, water
is tested weekly for nitrates and amonia and nothing. Water changed at
least once a week. temp has been maintained between 62 and 66 degrees.
The ich didn't seem to bother them that much no scrapping Or anything.
So this weekend went to pet smart to ask what I can do They suggested
"cure -ick" . Put it in yesterday and what do you know the happy fish
are not so happy anymore. All laying on the bottom very sluggish.
Usaully when I come to the tank they are like little aquatic puppies
Exteremly happy to see me(I know sound silly but it's true). Well
anyways they are all laying on the bottom with ten times as many spots
and a new over night case of tail rot. I'm going to change 20% of the
water but what the heck can I do. I think that it was a conspiracy by
petsmart so I have to buy new fish. I know what your going to say to
much fish for that tank, but if I am not able to cure these little
guys There will not be an oppertunity for a larger tank. I Have had
these fish for a year and half and I have grown quite attached to them
so please someone give some real advice!